To keep it short and ambiguous, my boss believes my second job is effecting my productivity. This may be true. I'm tired. My personal time is an unpredictably finite resource that seldom leaves me doing what I feel like doing. Taking on extra work isn't enjoyable. I'm doing it to get ahead for once.
Not only is my time my own business but if they really cares this much about optimizing my work they'd just pay me more. I'm not holding out on my potential for a raise or underperforming out of spite. I quite enjoy my job. But I'd sure enjoy it more if it were the only one I had.